1. Field of the Invention
This invention relates to polyethyleneterephthalate blends having high flexural strength, high humidity resistance, low mold shrinkage and improved impact strength and long-term heat resistance. More specifically, it relates to such blends which also contain an aromatic polycarbonate and a copolyester of terephthalic acid with ethylene glycol and cyclohexane-1,4-dimethanol.
2. State of the Prior Art
Various commercial fiberglass or mica filled PET compounds exhibit a number of weaknesses which may be attributed to the matrix resin. For example, these commercial materials have high mold shrinkage and warpage upon post-heating and low impact strength and brittleness upon even short term exposure to high temperatures. These defects are inherited from the PET matrix resin.
When particulate or plate-like fillers are added the mold shrinkage and warpage is partially reduced but there is a sacrifice in toughness, heat distortion and other mechanical properties such as tensile and flexural strengths. For example the commercial PET product sold under the trademark Rynite RE 5060 is filled with 35 percent of a combination of fiberglass and mica and the product sold under the trademark Rynite 530 has 30 percent fiberglass. In comparison, the mica-containing Rynite RE 5060 has less than 70 percent of the flexural, notched impact and tensile strength of the Rynite 530. Moreover, the mica-containing product has a heat distortion temperature of 400.degree. F. as compared to the 420.degree. F. of the Rynite 530.
Attempts to improve the impact strength of PET by blending with other high impact elastomeric or thermoplastic materials result in a lowering of the heat distortion temperature and also of the mechanical properties. For example, the commercial product Ropet R 400, which comprises an acrylic-PET blend, shows better low temperature toughness (1.8 vs. 0.8 ft-lb/in notched Izod impact) and less warpage than Rynite 530. However, the Ropet R 400 has both lower heat distortion (425.degree. F. vs. 435.degree. F.) and lower tensile strength (16,500 psi vs. 23,000 psi) than Rynite 530. Since both of these commercial products contain 30 percent fiberglass, it may be assumed that the property differences are derived from the matrix systems.
A number of U.S. Pat. Nos., for example, 4,105,150; 4,119,607; 4,172,859 and 4,195,000 disclose blends of PET with various elastomers or rubbery materials, including a hydrogenated monoalkenylarene-diene block copolymer, butadiene block copolymers, etc. However these compositions suffered substantial loss in stiffness and mechanical strengths.
U.S. Pat. No. 3,218,372 discloses a PET/PC binary blend. This composition has very poor flow properties when more than 7.5 percent of the aromatic polycarbonate is used, and moreover the blend has poorer heat resistance than the PET per se.